For practical applications of ultrafiltration (UF), an estimation of membrane fluxes under various operational conditions is very important. This study analyzed concentration polarization (CP) as a coupled transport problem with concentration-dependent solute viscosity. Besides the effects of variable viscosity, the model includes the effects of solute osmotic pressure, solute rejection at the membrane surface, and the axial pressure drop. This provides a fundamental understanding of the effects of various operating parameters on concentration polarization and transmembrane flux. A finite-difference solution of the transport equations is presented to model the concentration polarization in a thin-channel UF system. Simulation results for ultrafiltration of Dextran T-70 show that concentration-dependent solute viscosity adversely affects the transmembrane flux and needs to be carefully considered in modeling concentration polarization in membrane filtration.